1. Field of the Invention
This invention relates to methods for fabricating sold state emitters and in particular methods for tuning the emission characteristics of light emitting diodes coated by a conversion material.
2. Description of the Related Art
Light emitting diodes (LED or LEDs) are solid state devices that convert electric energy to light, and generally comprise one or more active layers of semiconductor material sandwiched between oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light. Light is emitted from the active layer and from all surfaces of the LED.
Conventional LEDs cannot generate white light from their active layers. Light from a blue emitting LED has been converted to white light by surrounding the LED with a yellow phosphor, polymer or dye, with a typical phosphor being cerium-doped yttrium aluminum garnet (Ce:YAG). [See Nichia Corp. white LED, Part No. NSPW300BS, NSPW312BS, etc.; Cree® Inc. EZBright™ LEDs, XThin™ LEDs, etc.; See also U.S. Pat. No. 5,959,316 to Lowrey, “Multiple Encapsulation of Phosphor-LED Devices”]. The surrounding phosphor material “downconverts” the wavelength of some of the LED's blue light, changing its color to yellow. Some of the blue light passes through the phosphor without being changed while a substantial portion of the light is downconverted to yellow. The LED emits both blue and yellow light, which combine to provide a white light. In another approach light from a violet or ultraviolet emitting LED has been converted to white light by surrounding the LED with multicolor phosphors or dyes.
One conventional method for coating an LED with a phosphor layer utilizes a syringe or nozzle for injecting a conversion material (e.g. phosphor) mixed with epoxy resin or silicone polymers over the LED. Using this method, however, the phosphor layer's geometry and thickness can be difficult to control. As a result, light emitting from different coated LEDs can vary, and light emitted at different angles can pass through different amounts of conversion material, which can result in an LED with non-uniform color temperature as a function of viewing angle. Because the geometry and thickness is hard to control, it can also be difficult to consistently reproduce LEDs with the same or similar emission characteristics.
Another conventional method for coating an LED is by stencil printing, which is described in European Patent Application EP 1198016 A2 to Lowery. Multiple light emitting semiconductor devices are arranged on a substrate with a desired distance between adjacent LEDs. The stencil is provided having openings that align with the LEDs, with the holes being slightly larger than the LEDs and the stencil being thicker than the LEDs. A stencil is positioned on the substrate with each of the LEDs located within a respective opening in the stencil. A composition is then deposited in the stencil openings, covering the LEDs, with a typical composition being a phosphor in a silicone polymer that can be cured by heat or light. After the holes are filled, the stencil is removed from the substrate and the stenciling composition is cured to a solid state.
Like the syringe method above, using the stencil method it can be difficult to control the geometry and layer thickness of the phosphor containing polymer. The stenciling composition may not fully fill the stencil opening such that the resulting layer is not uniform. The phosphor containing composition can also stick to the stencil opening which reduces the amount of composition remaining on the LED. The stencil openings may also be misaligned to the LED. These problems can result in LEDs having non-uniform color temperature and LEDs that are difficult to consistently reproduce with the same or similar emission characteristics.
Various coating processes of LEDs have been considered, including spin coating, spray coating, electrostatic deposition (ESD), and electrophoretic deposition (EPD). Processes such as spin coating or spray coating typically utilize a binder material during the phosphor deposition, while other processes require the addition of a binder immediately following their deposition to stabilize the phosphor particles/powder.
There has been recent interest in coating LEDs at the wafer level instead of the chip level to reduce the cost and complexity of fabrication. LEDs across a wafer can have different emission characteristics or color spread. FIG. 1 shows one example of a wavelength emission map 10 for a wafer of blue emitting LEDs showing wavelength variations across the wafer, and each wafer can have its own unique emission map. In the map shown, the wavelength distribution is in the range of approximately 445 to 460 nm, although other wafers can experience different distributions in different wafer areas. This distribution can result from different factors such as variations in the epitaxial material during growth of the LEDs, or from variations in the flatness (i.e. bow) of the growth substrate.
The wafer can be coated with a conversion material (i.e. phosphor) using one of the methods described above, and FIG. 2 shows a conversion material thickness map 20 following coating. In some fabrication processes the coating can be planarized using known methods. The thickness of the coating can vary across the wafer due to different factors such as variations in the thickness of the underlying wafer and in planarizing variations. In the embodiment shown the wafer experiences a total thickness variation of approximately 3 μm. The wavelength emission variations of the LEDs and thickness variations of the conversion material across the wafer can result in a spread of emission wavelengths or color points of the LED chips singulated from the wafer. This spread can exacerbate by phosphor loading variations or concentrations across the wafer.
The human eye is relatively sensitive to variations in emission wavelengths and can detect relatively small differences in emission wavelengths or color. Perceptible variations in color emitted by packages designed to emit a single color of light can reduce customer satisfaction and reduce overall acceptance of LED packages for commercial uses. In an effort to provide LEDs that emit light of the same or similar wavelength, the LEDs can be tested and sorted by color or brightness. This process is generally known in the art as binning. Each bin typically contains LEDs from one color and brightness group and is typically identified by a bin code. White emitting LEDs can be sorted by chromaticity (color) and luminous flux (brightness). Color LEDs can be sorted by dominant wavelength, and luminous flux, or in the case of certain colors such as royal blue, by radiant flux. LEDs can be shipped, such as on reels, containing LEDs from one bin and are labeled with the appropriate bin code.
FIG. 3 shows one example of a chromaticity region map 30 plotted on the 1931 CIE Curve, with each of these regions corresponding to a particular chromaticity of white LEDs. The regions are shown surrounding the black body curve or black body locus (BBL) and each of these regions is designed to designate chromaticity variations that are within acceptable ranges to the human eye. For example, region WF designates a particular region having substantially imperceptible chromaticity variations such that LEDs emitting within this region would be binned together.
FIG. 4 shows one example of the distribution of emission characteristics for a sample batch of wafers with blue emitting LEDs, following coating with a conversion material. The region designations correspond to different chromaticity regions for a map, such as the one in FIG. 3. The majority of the coated LEDs emit in regions WC, WD, WG and WH, with the remaining LEDs emitting in other regions, some being outside the map regions. This variation in emission characteristics results from emission wavelength variations across the LED wafer and phosphor thickness variations, and the emission variations would require multiple different bins for the individual LEDs.
This binning process typically increases the manufacturing cost of LEDs by the overhead associated with the testing and separation of devices with different emission characteristics, and the formulation of the data and records surrounding this process. The greater the number of bins for a particular LED being manufactured, the greater the additional cost associated with the binning process. This in turn can result in increased end cost for the LEDs. This binning process could be reduced if coated LEDs across the wafer emitted light closer to a target color point.
One method for measuring the target emission for LEDs is by standard deviation from a target color point, with one example being deviation by MacAdam Ellipses on the CIE color region map as shown in FIG. 3. These ellipses are generally known in the art and are defined to establish the boundaries of how far colors of light can deviate from the target before a difference in the target light is perceived. MacAdam ellipses are described as having “steps” or “standard deviations”. For example, any point on the boundary of a “1-step” ellipse drawn around the target represents one standard deviation from the target. Specified tolerances for conventional lamps (incandescent or fluorescent) are within a 4-step MacAdam ellipse. For LEDs to become more generally accepted by consumers for general lighting applications, they should be provided with emission characteristics within accepted specified tolerances, such as the 4-step MacAdam ellipse. For some current manufacturing processes, the yield within a 4-step MacAdam ellipse can be 20% or lower.